Hearing aid



FIG. 1 is perspective view of the hearing aid, showing a side, elevated perspective of the ear unit of the hearing aid and a side, elevated perspective of the behind-the-ear unit of the hearing aid;

FIG. 2 is a view of the hearing aid, showing a side, elevated perspective of the ear unit and a top view of the behind-the-ear unit;

FIG. 3 is a view of the hearing aid, showing a front perspective of the ear unit and a right side view of the behind-the-ear unit;

FIG. 4 is a view of the hearing aid, showing a bottom perspective of the ear unit and a bottom view of the behind-the-ear unit;

FIG. 5 is a view of the hearing aid, showing a side view of the ear unit and a rear view of the behind-the-ear unit;

FIG. 6 is a view of the hearing aid, showing a rear perspective of the ear unit and a left side view of the behind-the-ear unit; and,

FIG. 7 is a view of the hearing aid, showing a side, bottom perspective of the ear unit and a front view of the behind-the-ear unit.

The broken lines form no part of the claimed design. 

The ornamental design for a hearing aid, substantially as shown and described. 